 Attempting to turn on the engine can cause significant damage. Lift the hood, disconnect the battery while wearing dry gloves to prevent electrical shorts and further damage to the car's complex electrical system. Number two, look for a watermark on the car's exterior and interior. Number three, document everything with photos and videos. Of course, you want to contact insurance, use a wet dry vacuum to remove standing water, get a professional mechanic to inspect it, then a professional cleaning service.